I love eating cold pizza leftovers straight from the fridge. Someone recently told me that you should always reheat leftover pizza to kill off any bad bugs that could be growing in there. Is there any scientific basic to this? I've never got sick so far.
Not really. Assuming that the pizza has been cooled down and stored properly (for which see here and here) then it most likely won't have developed a potentially harmful microbial load. Additionally, while reheating might kill off most of the microbes in the food (assuming that you reached and maintained a temperature sufficient for pasteurization, which isn't a given) that still wouldn't render the food completely safe, because even dead microbes can leave behind toxic substances that can make you sick.
Be warned that just because you haven't gotten sick in past, that doesn't mean your method is completely safe. Food safety is a science of relative risk, not absolute certainty, and getting lucky 99 times in a row doesn't mean that the 100th time won't be a miserable one.
But, bottom line: put your pizza into the fridge when you're done eating it, and don't keep it for more than a couple days or if it shows obvious signs of spoilage, and most likely you'll be okay. Reheating is not essential.
The test of a truly great pizza is how it tastes the next day — cold, right out of the refrigerator.
Provided the freshly cooked pizza was not left out at room temperature for an extended period of time; and it's been refrigerated for only a day or so; it ought to be safe to eat.
It's a different experience eating cold pizza, but if it was made with the right ingredients — sauce, cheese, anchovies, etc. — the flavors can come through in a way that can make it quite a delicacy in its own right.
A lousy pizza will not taste as good cold, so reheating may be necessary to crisp the crust and remelt the cheese, to possibly make it more edible.
Here in Italy it is normal to buy in the stores cold pizza for eating as snack. Pizza is similar to bread so it should be safe if conserved properly.